It is planned to carry out the proposed research along two lines. It has been demonstrated that antigonococcal antibodies attached to sheep erythrocytes are capable of detecting the presence of small numbers of gonococci, living or dead, in aqueous suspensions. This procedure will be tried on clinical specimens to determine if some of the members of the normal flora of the genitourinary tract may give false-positive reactions. Also a card technic instead of the more expensive and sophisticated microtiter technic will be tried. It is planned to infect laboratory animals with Mycoplasma arthritidis and, when arthritis develops, test the fluid of involved joints for mycoplasma by culture and by the indirect hemagglutination test using erythrocytes sensitized with antimycoplasmal antibodies.